Dispersants are commonly added in small amounts to ensure that inorganic or organic fine powders are uniformly dispersed in water or solvents so that the resulting dispersions have low viscosity and high flowability. Improved dispersibility will contribute not only to an improvement in the quality of the final product or its workability or handling property but also to an improvement in economy through the increase in the concentration of dispersions. Hence, dispersibility is a very important factor in various fields, to say nothing of the importance of the performance of the dispersants to be employed.
Sodium hexametaphosphate has been used for many years as a dispersant for pigments to be incorporated in coated paper. Sodium polyacrylate is known as an effective dispersant in a broad range of fields. Copolymers such as those of .alpha.-olefins and maleic anhydride and those of styrene and maleic anhydride have also been used as dispersants for pigments after they are converted to water-soluble salts.
However, none of the dispersants available today are completely satisfactory in terms of dispersing ability and aging stability.
In order for cement mixes (e.g. cement pastes, mortars and concretes) to have improved workability, they must exhibit high initial flowability and experience small change in flowability with time. The flowability of cement mixes is known to decrease with time due primarily to hydration reaction of cement with water and to physical aggregation of cement particles. Upon mixing the ingredients, hydration reaction of cement with water occurs in cement mixes and as time lapses, physical and chemical aggregation of cement particles proceeds to reduce cement flowability and its workability and constructibility will consequently decrease with time. This phenomenon is commonly referred to as "flow reduction" in mortars or "slump loss" in concretes and limits the "open time" of cement mixes. In the case of fresh concretes, slump loss causes various problems such as limited time of transportation, prolonged standby on the installation site and temporary interruptions of transfer by pumping, all these phenomena leading to undesirable effects such as deterioration of quality and low operational efficiency. Slump loss is also deleterious to fabricated concrete products since it limits molding time or causes insufficient centrifugal re-compaction. Therefore, slump loss, or time-dependent decrease in the flowability of cement mixes such as cement pastes, mortars and concretes, is a critical problem to be solved.
Several proposals have so far been made with a view to preventing slump loss in concretes. Japanese Patent Publication No. 51-15856 shows a method in which a concrete admixture selected either from water-soluble salts of sulfonated aromatic compounds or from water-soluble salts of the products of condensation between sulfonated aromatic compounds and formaldehyde is added repeatedly to concrete to maintain its flowability for a prolonged period. This method is effective to some extent but not only is it low in operational efficiency due to cumbersome procedures of addition of the admixture but it is also disadvantageous from an economic viewpoint.
Another method which involves the addition of a retarder such as an oxycarboxylic acid causes reduction in strength and insufficient hardening. Hydration of cement can be retarded by this method but it is very difficult to prevent physical aggregation of cement particles.
Japanese Patent Public Disclosure No. 54-139929 shows a method in which granules of a concrete admixture such as the product of condensation between naphthalenesulfonic acid and formaldehyde are added to concrete and dissolve slowly to prevent slump loss in the concrete. This method is effective to some extent in preventing slump loss but on the other hand, it is difficult to control the rate of dissolution of the admixture and residual admixture granules will reduce the strength and durability of the concrete.
Japanese Patent Publication No. 63-10107 (Japanese Patent Public Disclosure No. 60-11256) shows additives composed of a saponified styrene-maleic acid copolymer that can be added to cements to prevent slump loss. These additives, however, are not very effective in improving the flowability of cement mixes. Besides, they entrain an undesirably large amount of air and suffer from an economic disadvantage in that they have to be prepared from expensive materials.
Thus, none of the methods so far proposed to prevent slump loss are far from being satisfactory for practical purposes.